Method of and apparatus for reducing air pollution in the thermal processing of ores and other materials

ABSTRACT

Off-gases from the high temperature firing of predominantly inorganic material, as in the manufacture of sinter and cement clinker, calcining and heat-hardening of pellets and other preforms, which have become fouled with various fumes, organic and/or metallic vapors, etc. by being used to dry and preheat the raw material with such fuel and other ingredients as may be mixed with the raw material, are recycled to a zone where they again pass through the highly-heated hot product to incinerate any burnable fumes or other contaminants in the recycled gases. All or part of such gases are then exhausted to the atmosphere while any part of said gases that are not exhausted mix with and are recycled along with the off-gases being currently generated in the process. Shaft furnaces, traveling grate pelletizing and sintering bands, and combination systems are constructed for such recycle of the contaminated off-gases.

United States Patent [191 Porteus [451 May 8,1973

[54] METHOD OF AND APPARATUS FOR REDUCING AIR POLLUTION IN THE THERMALPROCESSING OF ORES AND OTHER MATERIALS Mahony ..263/53 R PrimaryExaminer-John J. Camby Attorney-William H. Parmelee [57] ABSTRACTOff-gases from the high temperature firing of predominantly inorganicmaterial, as in the manufacture of sinter and cement clinker, calciningand heat-hardening of pellets and other preforms, which have becomefouled with various fumes, organic and/or metallic vapors, etc. by beingused to dry and preheat the raw material with such fuel and otheringredients as may be mixed with the raw material, are recycled to azone where they again pass through the highly-heated hot product toincinerate any burnable fumes or other contaminants in the recycledgases. All or part of such gases are then exhausted to the atmospherewhile any part of said gases that are not exhausted mix with and arerecycled along with the off-gases being currently generated in theprocess. Shaft furnaces, traveling grate pelletizing and sinteringbands, and combination systems are constructed for such recycle of thecontaminated off-gases.

13 Claims, 5 Drawing Figures PATENTEB MAY 8 ms SHEET 1 OF 2 METHOD OFAND APPARATUS FOR REDUCING AIR POLLUTION IN THE THERMAL PROCESSING OFORES AND OTHER MATERIALS This invention is for a method of and apparatusfor the high temperature firing of various inorganic substances toreduce the atmospheric pollution which results from present practices,but without appreciable impairment of the thermal efficiency.

In many industrial processes, such as beneficiation of ores by sinteringor pelletizing, or the manufacture of cement clinker or the burning oflime, to mention some of them, material undergoing treatment is heatedto a high temperature, often above 2,000 F. This may involve the use ofan endless traveling grate or sintering band, or a shaft furnace, or acombined endless grate and kiln, or the combination of an endless grate,a rotary hearth and shaft furnace. In some such processes fuel may becombined with the material being processed; at other times hotcombustion gases are generated with fuel burners that produce therequired heat, and sometimes there may be a combination of burner meansand fuel mixed with the material being processed. Also, in some casesthe inorganic material being processed may produce an exothermicreaction to provide a substantial part of the heat requirements of theprocess. In all such procedures forced circulation of combustion air andthe products of combustion is effected by the use of hot fans orblowers, and in most cases heat is recuperated by the use of hot gasesresult- I ing from the high temperature firing of the product to preheatand dry the green, raw or unfired material. Also, in many cases heat isrecuperated in the combustion process from air that has been used tocool the hot product or material after it has been fired.

In all such processes of which I am aware, hot gases from which heat isrecuperated by drying and preheating the raw or green material aredischarged into the atmosphere, either directly or after passing througha dust collector for the removal of particulate material. Thus, forexample, in a shaft furnace where green pellets are charged into the topof the shaft tired to indurating temperature intermediate the top andbottom and then cooled by an updraft of air through the previously-firedhot pellets in the lower portion of the shaft, that air, heated bycontact with the hot pellets, is utilized in the firing zone and thehardened cool pellets are discharged at the bottom of the shaft. The hotair and gases from the firing zone rise from the firing zone into theoverlying body of green pellets where it preheats and dries the greenpellets and such gases are then discharged into the atmosphere.Likewise, in the heatinduration of green pellets on a traveling grate,air may be forced updraft through the fired pellets in a cooling zone,and thereby becomes heated. It then flows into a firing zone where itsupplies combustion air and the air and gases pass downdraft through thebed of pellets on the traveling grate. These hot gases are then againpassed through the green pellets at the charging end of the travelinggrate to dry and preheat the green pellets, after which they aredischarged into the atmosphere.

As a third example, in the sintering of iron ore, a deep bed of combinedore and fuel is continuously deposited on a hearth layer at the chargingend of a traveling grate or sintering strand and the bed is carriedfirst under an ignition hood where burning gases from burners above thebed are forced down through the bed to ignite the fuel in the top of thebed. From the ignition zone the grate carries the bed over a successionof windboxes and air is drawn down through the bed so that the burningof the fuel, referred to as the flamefront progresses further andfurther toward the bottom of the bed. The air, flowing through the hotbed above the flame-front, cools the bed and recuperates the heat foruse at the flame-front while the hot combustion gases and unburned airpassing through the bed below the flame front preheats and dries theportion of the bed below the flame front. All of the air and combustiongases so used or generated are discharged into the atmosphere.Particulate material may be removed by conventional dustcollectingapparatus.

The foregoing are representative of various high temperature firingprocesses with which the present invention is concerned, but by no meansinclusive of all such processes. The point is that gases passing throughor generated in the hottest or very hot zones of the apparatus flowimmediately into contact with the green material and are thendischargedinto the atmosphere. ln flowing through the green material to dry andpreheat it, these gases collect not only moisture, but pick up volatilesfrom fossil fuels such as oil and coal, partial oxidation products ofvarious organic bonding or additive materials, fumes from oil or otherorganic wastes that may be present in shop or other wastes, and metallicvapors of low melting point metals, as lead and zinc that are frequentlymixed with or comprise a part of the charge, as well as both combustibleand non-combustible particulate materials, such as coke breeze, oredust, coal dust and the like, depending on what is being burned or wherethe process is being practiced. The particulate material may be removedin conventional dust collectors, but the volatiles and metal or fuelvapors and other fumes are discharged into and pollute the surroundingatmosphere.

The present invention proposes the modification or redesign of apparatusas presently constructed, and the improvement in methods as presentlypracticed to circulate all air or gases used in drying and preheatingthe material being fired into the very hot zones where combustible andmetallic vapors and fumes are incinerated, temperatures ordinarily beingwell above 2,000F., and particulate substances may be largely burned orfused into the hot product, thus reducing, if not practicallyeliminating in some cases, the burden of particulate solids dischargedinto the atmosphere, as well as eliminating most objectionable fumes andvapors. Only sulphur dioxide which may be present in objectionableamounts in some cases remains for removal by further conventionalprocessing. Taking the examples above cited, in the shaft furnace airand products of combustion used to dry and preheat the layers of chargeabove the firing zone are recirculated to the bottom of the shaft torise eventually into the firing zone where the highest temperaturesprevail, and where incineration of volatiles and combustible pollutantstakes place. From here the gases are cleansed and freed of combustibles,or at least some substantial percentage of them, are withdrawn andexhausted to the atmosphere, usually through a dust collector forremoval of entrained particles.

In the pelletizing example above outlined, gases from the drying andpreheating zones are recirculated either directly to the hood over thefiring and indurating zone to pass through the bed where it is at anincinerating temperature, and/or circulated to the cooling zone and thento the firing and indurating zone. In either case gases are dischargedto the atmosphere only after they have been thus recycled through thehighly-heated bed to incinerate the impurities which they acquired indrying and preheating other pellets.

Likewise, in the sintering apparatus a hood is placed over the sinteringstrand near the discharge end and gases and air from the windboxes aheadwhich have passed down through the bed to dry and preheat the bed belowthe flame front are conducted to this hood to pass down through theincandescent sinter where any remaining material below the flame fronthas already been so thoroughly dried and preheated as to yield nofurther, or at least for all practical purposes, no further products tobe incinerated. In so passing down through this terminal portion of thebed where perhaps the highest temperatures prevail, fumes and volatilesthat have been acquired in the use of the gases for drying andpreheating the bed are incinerated, whereupon the gases are thenexhausted directly to the atmosphere,-

preferably, as in all cases, through a dust collector.

In addition to substantially reducing atmospheric pollution, it isexpected that the process will improve the economy of the apparatus byreduction of power requirements for the fans used for circulating theair and gases.

My invention may be more fully understood by reference to theaccompanying drawings in which several forms of apparatus areschematically illustrated, the invention residing in the gas flowarrangements while the specific apparatus details per se are well known.In the drawings:

FIG. 1 illustrates schematically a shaft furnace constructed to embodythe apparatus and practice the method of my invention;

FIG. 2 illustrates schematically an endless grate constructed to embodythe apparatus and practice the method of my invention forheat-induration of pellets and other formed bodies of ore or othermaterial;

FIG. 3 illustrates schematically the invention as applied to an endlessor traveling grate employed for sintering ore, burning cement clinker orlime, the manufacture oflight-weight aggregate and the like;

FIG. 4 illustrates schematically the invention as applied to agrate-kiln apparatus and method for firing pellets or other agglomeratesand other processes in which lumps or bodies of rock, ore, minerals orthe like are fired to indurate, calcine, sinter or otherwise processthem at high temperatures; and

FIG. 5 illustrates schematically the invention as applied to a so-calledHeat Fast or similar system of pelletizing using in combination atraveling grate, a rotary hearth and a shaft furnace.

Referring first to FIG. 1 there is a shaft furnace 2 of a type wellknown in the art having a hood 3at the top and a discharge cone 4 at thebottom. Pellets or other preforms to be fired, such as briquettes,agglomerates or other bodies, usually having admixed therewith a fossilfuel such as powdered coal or oil, are introduced into the top of thefurnace through an air-lock of some kind, here schematically indicatedas a star-wheel feeder 5, from a hopper 6. Fuel burners 7 intermediatethe top and bottom of the furnace supply hot combustion gases and air tothe mid section of the furnace to travel upward through the furnace andeffect the burning of the fuel in the pellets or other bodies in thefurnace. At a level usually more than twelve to 14 inches below the topof the bed the volatiles in the fuel will have been consumed and thepellets will be at or near an indurating temperature above 2,000 F. andin all cases well above the 1,400 F. 1,500 F. level required for fumeincineration. Fuel and combustion air supplied to these burners areindicated at 8 and 9 respectively, and 10 indicates tuyers or othermeans for introducing the hot gases into the shaft.

Near the bottom of the furnace 2 are perforated cones or other shaft airinlet means 11. In the usual process of operating a shaft furnace therewould be no air seal at 5 and the hot gases rising through the layer ofgreen pellets above the firing zone would preheat the pelletsimmediately above the firing zone and dry the green pellets at the topand be vented to atmosphere at the top of the furnace.

With the present invention there is a duct 12 which may include a fan 13that circulates gases and air from the top of the furnace to the airinlet cones in the lower portion of the furnace. At a level of thefurnace where the temperature of the pellets will have reached 1,600 F.or more and at least twelve to 14 inches-and preferably morebelow thetop of the uppermost layer of pellets in the furnace there are one ormore perforated refractory ducts 14 through which gases and air in whichall burnable impurities have been inceratedwhich should occur at around1,400 F.is withdrawn into duct 15 and dust collector 16 and thendischarged into a stack (not shown). There is a damper indicated at 17to regulate the volume of air and gases withdrawn in relation to theamount which remains in the furnace to rise through the overlying chargein the zone indicated by the bracket a to effect preheating and dryingof this portion of the charge. It is this portion of the gases used forpreheating and drying that becomes contaminated and which has heretoforebeen discharged into the atmosphere. With the present invention onlygases and air that have traveled up through the hottest part of thecharge in the furnace and hence which are most free of objectionablegases and vapors are exhausted into the air.

Assuming, for example, that combustion air is 10,000 SCFM and shaft airis 15,000 SCFM, the process as heretofore practiced would discharge all25,000 SCFM to the atmosphere, whereas with the present invention 10,000SCFM could be withdrawn into duct 15 and 15,000 SCFM recirculated,requiring only a make-up of 10,000 SCFM. While the present process mightlose some heat in the hot gases withdrawn through pipe 15 which wouldhave been largely recuperated, at least theoretically, the reduced powerrequirements for bringing into the system only 10,000 SCFM, and thereduced heat loss with the discharge of only 10,000 SCFM as against25,000 SCFM more than offsets the heat loss which results fromdischarging a part of the very hot gases directly to the stack withoutrecuperation of the heat.

l have described the process as being one where the pellets or othercharge may have fuel mixed therewith, but this might not always be thecase, and in such case all the heat would be supplied from the burners7. Also, in some cases the process might be one in which pellets arereduced from a higher oxide to a lower one or partially at least,reduced to metal, but the process insofar as circulation of gases andair would be the same, as would also be the case with calcining lime orother high temperature shaft furnace operations.

It is to be noted that in this, as well as the processes to behereinafter described in detail, the heat of the product being processedinduces incineration or cleaning of non-solid pollutants from the wastegas and no after-burner, as required by many processes, is needed.

In FIG. 2 there is schematically indicated an endless traveling gratetype of apparatus commonly used in the indurating of pellets and otheragglomerates and preforms. The apparatus designated generally as has thepass line of the endless grate designated generally by the line A-A. Thetravel of the endless grate is in the direction of the arrow shown inFIG. 2. Below the endless grate at the charging end there is a windboxarea 21 which is for updraft drying; a second windbox area 22 followsthis, and is for downdraft drying, preheating, and may include a portionof the firing zone. Next there is a windbox area 23 that is below themajor portion of the firing zone and the so-called after-firing orindurating zone. Following this is a windbox area 24 for the principalor first updraft cooling zone, and beyond this is a windbox area 25 forfinal cooling. Above the traveling grate there is a hood 21A for theupdraft drying gas; 22A over the downdraft drying and preheating zones,and a hood 23A over the firing and indurating zones. There is a hood 24Aover the first cooling zone and a hood 25A over the final cooling zone.Under the hood 23A there may be a series of fuel burners 26. Thisarrangement is more or less typical of traveling grate pellet firingapparatus and is of the general type shown in U.S. Pat. No. 3,172,754.All details such as the number of windboxes in each area, the provisionof dampers, the design of seals, etc. are well known in the art.

The hottest gases that are generated in the operation of the apparatusare discharged into the windbox area 23 below the firing and induratingzone. Customarily some of these gases are carried forward to circulateup through the updraft drying zone and from the updraft firing zone theyare generally discharged into the atmosphere. Some of these gases mayalso be used in the downdraft drying zone, while some may be dischargeddirectly to the atmosphere. According to the present invention thehighly-heated gases from the windbox area 23 are conducted through duct27 to the windbox area 21 in the updraft drying zone. From the updraftdrying zone these gases are conducted through a passage 28 to thewindbox area 24 in the first cooling zone. A damper at 29 may controlthe flow of hot gases through the duct 27. Gases and air discharged fromthe windbox area 23 not used for updraft drying may be discharged to theatmosphere through pipe 30 in which there is a damper 31. This pipe maydischarge into a dust collector 32 of any known preferred type whereparticulate material is removed, and from which the cleaned gases passto the stack (not shown). Tempering air may be introduced into line 27,as indicated at 27a and damper 27b.

The hot gases that pass through the duct 27 to the windbox area 21 ofcourse are cooled in passing through the green pellets at the chargingend of the grate, and in addition they become polluted with variousvapors and gases as previously described. They may therefore be used inthe initial cooling zone where they contact the hottest pellets. To someextent impurities carried in these gases and air up through the bed inthe first cooling area will be incinerated or consumed. However, all ofthe gases and air from the hood 24A are carried forward through duct 33and discharged into the hood 23A over the firing and indurating zone.Here they pass down through the bed of pellets in the vicinity of theportions of the bed where temperatures are well above the incineratingtemperature of any gases or combustible vapors still remaining in therecirculated gases to thereby completely remove these impurities. Gasesfrom the windbox 22 which also include some vapors and fumes as well asparticulate matter from the drying and preheating operation arecirculated through the duct 34 for recycle either to the drying andpreheating through branch pipe 35, or into the firing and induratingzone through the branch pipe 36. Dampers at 37 and 38 in the pipes 35and 36 respectively regulate the volume of flow through these two lines.Other parts of the apparatus, as for example the final cooling zone, arenot particularly relevant to the invention which embodies thecirculation or recycling of gases used for drying and preheating throughan incinerating zone before discharging them into the stack system.-

Make-up air may be introduced in part as primary air to the burners 26in the firing zone or as tempering air into the line 33, or may beelsewhere introduced as required, but here again, while there may be alarge volume of gases and the air circulating through the system, themakeup air need only balance the amount which is bled through line 30 tothe stack, except for leakage.

FlG. 3 discloses the invention as applied to a sintering strand of thetraveling grate type. The sinter bed in this figure is confined betweenthe lines CC at the top and D-D at the bottom. Generally the bedcomprises the material to be burned or fired or sintered, such as ore orfrom cement clinker-forming ingredients or possibly limestone or thelike to be calcined. The raw material combined with fuel is loaded ontothe traveling grate at the right end as viewed in FIG. 3 and carriedtoward the discharge end at the left of the figure. Over thecharge-receiving end of the bed on the traveling grate there is anignition hood 40 having a burner 41 to which air is supplied by a blower42, and to which gases or other fluid fuel is supplied through pipe 43.Below the grate there are a series of windboxes, each of which isdesignated M, and each of which communicates through passage 45 to acommon outlet duct 46. Dampers are indicated at 47 to regulate the flowof gases to each windbox. The sinter bed beyond the ignition hood 40 isopen to the ambient air. Normally this exposure to the ambient air wouldextend to the discharge end of the grate, but according to the presentinvention there is a hood 48 over the bed at the discharge end and thereare separate windboxes 49 under this portion of the bed leading throughpassages 50 in which there may be dampers 51 to a common duct 52 whichterminates in a suction fan 53. The discharge 54 of the suction fanleads to a conventional or preferred dust collector not shown, but whichwould appear schematically as the dust collectors do in either FIGS. 1or 2, and from the dust collector the gases are delivered to the stackfor discharge into the air.

With the present invention the duct 46 leads into a duct 55 which opensinto the hood 48.

The sinter bed varies in depth in different plants but may be of theorder of 2 feet in depth. As the bed travels under the ignition hood thefuel at the top of the bed is ignited and the burning gases travel downthrough the bed to preheat and dry the underlying bed of green materialand fuel before being drawn into the windboxes 44. As the burningportion of the bed progressively moves along over the succeedingwindboxes, atmospheric air drawn down through the bed sustains thecombustion on a gradual downward slope as indicated by the broken line56 so that the air drawn into the suction boxes 44 is increasingly usedto cool the burned sinter above the diagonal line 56 which is generallyreferred to as the Flame Front, and decreasingly preheat and dry the bedbelow the flame front which is of course of gradually reducing depth. Bythe time the flame front reaches the area covered by the hood 48 theremaining portion of the bed will have been completely preheated anddried and will be at its highest temperature approaching that of theflame front of 2,350 F. to 2,450 F. With the arrangement and processhere illustrated, all of the gases and air that enter the windboxes 44and which will be contaminated with vapors and smoke from the fueland/or with other combustible vapors or compounds will be passed throughthe bed under the hood 48 where the highest incinerating temperaturewill be encountered, resulting in the burning of the oxidizable productsand the incineration of impurities. Even particulate matter, as in theother processes, may be fused in this zone to the previously-formedsinter, reducing the amount of particulate matter which passes throughthe fan to the dust collector.

Here again all gases used in preheating and drying the bed pass througha flame front or high temperature zone well above an incineratingtemperature for the impurities in the gases before being discharged tothe atmosphere. The ignition zone and the entire length of the flamefront may be hereinafter sometimes referred to as the firing zone orhigh temperature processing zone.

FIG. 4 discloses the application of this invention to the so-calledgrate-kiln process commonly used for hardening pellets and otherpreforms or agglomerates. In this apparatus the pellets are fed onto atraveling grate designated 60 which moves over windbox areas 61 and 62and under hoods 63, 64 and 65, hood 63 being over an updraft zone, andhood 64 is over a downdraft drying and preheating zone, and hood 65 isover a preheating zone. The preheated product from the discharge end ofthe traveling grate passes into the receiving end of a rotary kiln 66where the pellets are fired to the maximum temperature 'and indurated.From the rotary kiln 66 the fired pellets are discharged into a firstcooling zone 67 and then into a second cooling zone 68, and they aredischarged at 69. A fuel burner designated 70 having a blower and a fuelsupply designated 71, fires directly into the rotary kiln 66. Combustiongases and air from this burner are withdrawn at the inlet end of thekiln into the hood 65 over the preheating zone. The hot gases are drawndown through the bed of pellets on the traveling grate to preheat them,the gases flowing into the windbox 61 to pass updraft through the dryingzone. A portion of these gases are withdrawn through duct 74 fordischarge into the hood 64 for downdraft drying of the bed of pellets,the gas so circulated flowing through the bed of pellets back into thewindbox 62.

In a system of this kind as heretofore operated, the gases leaving thehood 63 and bearing all of the impurities picked up in the preheatingdowndraft and updraft drying would be discharged into the atmosphere.According to this invention a duct 75 leading from the hood 63 carriesthe gases and air from the hood 63 to a fan 76 which forces them upthrough a windbox 77 under the first cooling zone 67. These gases thenflow, as indicated by the arrows, into the kiln 66 where they minglewith the combustion gases from the burner 70 and contact the hot pelletsin the kiln and in the discharge end of the preheating zone. A portionof these gases will of course flow with the combustion gases downthrough the bed of pellets in the preheat zone as previously described,but an offtake conduit 78 in which is a damper 79 will bleed part ofthese gases as well as part of the combustion gases from the system tothe atmosphere. Pipe or conduit 78 will carry the gases which arewithdrawn through it to a conventional or preferred form of dustcollector as previously described, and from the dust collector they willbe discharged into a stack and thence into the atmosphere.

Here again it will be seen that all of the gases which have been used topreheat and dry the pellets and which have accordingly becomecontaminated by contact with the green pellets will pass through thehottest zone of the furnace to incinerate any combustible gases or otherproducts and be discharged then to the atmosphere where they arerelatively innocuous. Cooling air supplied to the windbox under thesecond cooling zone 68 is exhausted directly to the atmosphere.

FIG. 5 shows the invention as applied to a heatfast" pellet burningoperation.

The pellets are first charged into a traveling grate designatedgenerally as 80. They are then conveyed onto a rotary hearth designatedgenerally as 81. They are discharged from the rotary hearth into the topof a shaft furnace designated generally as 82, and finally aredischarged from the shaft furnace in a fully burned condition.

More specifically, green pellets or other preforms or bodies to be firedare charged onto the traveling grate 80 at the right end as viewed inFIG. 5. In the first portion of their travel on the grate the greenpellets are carried through an updraft drying section indicated bywindbox 83 and hood 84. They move then into a second drying section,also preferably an updraft drying and preheating section, indicated bywindbox 85 and hood 86. The arrangement is such that dried and initiallypreheated pellets are deposited on a rotary hearth structure 81 having arotating hearth 87 that carries them through an enclosure which issupplied with heat from fuel burners 88, the hot gases from whichcirculate counter to the direction of rotation of the hearth. Here thepellets are preheated to a temperature of the order of 2,000 F. inaccordance with usual practice. From the discharge end of the enclosure;the

preheated pellets are charged directly into the top of the shaft furnace82 through an air-lock, as schematically illustrated in FIG. 1. Externalor internal burners schematically indicated at 89 quickly bring thepellets in the upper portion of the shaft furnace to induratingtemperature, usually in a range between 2,400 F. and 2,450 F. Theoperation of course is a continuous one, as it is in the other methodsherein described.

The overall organization of the grate drier, the rotary hearth and theshaft furnace is known, but as heretofore operated the gases from thedrier and preheater have been discharged directly into the atmosphere.According to the present invention offgases and air from the top of theshaft furnace are removed and part of them are bled off into duct 90 andconducted to a dust collector for removal of particulate impurities andthen discharged into the stack for discharge into the air. The dustcollector is indicated at 91, and 92 is a damper to regulate the flow ofgases to the stack.

The rest of the gases and air from the top of the shaft furnace areconducted through duct 93, in which is a flow-regulating damper 94, tothe windbox 85 of the grate-drier unit. After passing up through the bedof pellets, this stream of gas and air flows from hood 86 through duct95 to shaft air inlet cones, as shown in FIG. 1 for example, in thelower part of the shaft furnace 82. Inside this furnace the gas and airso discharged move up through the shaft furnace and the pellets wherethey reach their maximum temperature, thus insuring the incineration ofcombustible pollutants before any part of the combustion gases or air isdischarged to the stack.

At the lowermost part of the shaft furnace there is a cooling air inletduct 96 and above it a cooling air outlet duct 97. Duct 97 carries airand gases that have been heated by contact with hot pellets in passingupwardly in the lower part of the shaft furnace and conveys primaryand/or secondary combustion air to the burners 88 of the rotary hearthunit. Gases and air are removed from near the entering end of the rotaryhearth enclosure through duct 98 and discharged into the windbox 83 ofthe grate drier to dry the incoming green pellets and be cooled therebyas they pass upwardly through the bed of pellets on the grate into hood84. The cooling gas inlet duct 96 conducts this cooled flow of air andcombustion gases to the lower portion of the shaft furnace as abovedescribed. A damper or other flow-regulating device 99 in pipe 97diverts some of the air and gas flow in this part of the system up intoand through the shaft furnace to be incinerated and flow out the top ofthe shaft furnace to escape to the atmosphere or recirculate through thesystem as heretofore described. Blowers or fans will be used inaccordance with usual practice to secure the required flow of gases andair. The location of any such fan or fans will be selected to bestperform their purpose, as will be well understood by those skilled inthe art, but for the purpose of completeness of this disclosure one fanhas been indicated at 100 in line 95 and at 101 in line 96.

Ill)

From the foregoing specific embodiments of the invention and the methodsof practicing the same, it will be seen that it involves apparatus thecomponents of which have been heretofore successfully used. However,according to this invention where heated offgases that have heretoforebeen used to dry and preheat the pellets or other material beingthermally processed, and which have become objectionable from being soused are not now discharged into the stack without first being recycledthrough a high temperature zone and then entirely or in part exhausteddirectly to the atmosphere through some sort of dust collector. By beingso recycled and heated the polluting gases and vapors are burned orincinerated. In this process some entrained solid particles in thenature of ash or dust may be fused onto the incandescent product whichis being processed and thereby reduce the required capacity of theconventional dust collectors. Wherever applicable the term pellets willinclude briquettes or other formed bodies, and the term sinter willinclude clinker, calcined material, or fused ceramic bodies. Thus, inthe same manner, the invention is applicable to cement and lime kilnswhere hot combustion gases have heretofore been used to dry and preheatthe raw material, and wherein burning and cooling may be effected invarious types of apparatus, including apparatus as herein disclosed orother conventional apparatus and as rotary kilns where the firing occursin one kiln and cooling in another. In addition, in the severalprocesses as herein described, there is a maximum of gas recirculationwith a minimum of exhaust, so that as specifically pointed out inconnection with FIG. 1 but inherent also in the other embodiments, lossof heat from exhausting some of the hottest gas to the stack is offsetby the reduced energy requirements of the fans and possibly a moreeffective recuperation of heat from the body of circulating gases andair which are being recirculated.

I claim:

1. In the thermal processing of predominantly inorganic solid materialwhich is tired to a temperature in excess of l,600 F. by a process whereheat is recuperated from hot gases that have been discharged from theheat processing of said material by passing said hot gases through abody of unfired material about to be heat processed to thereby dry andpreheat the unfired material whereby the gases so used becomecontaminated with fumes and vapors, the steps which comprise:

a. recycling all gases so used through a body of the inorganic materialwhere it is at a temperature of at least l,600 F. to incinerate anycombustible contaminants contained therein, and

b. discharging at least a portion of the gases so recycled to theatmosphere before said gases may again contact material which has beenso preheated.

2. The process defined in claim 1 wherein the portion of the gases sodischarged to the atmosphere is removed directly from an area of the bedwhere the temperature is above l,600 F.

3. The method defined in claim 2 in which a portion only of the recycledgas is discharged to the atmosphere and the remainder, mingled with hotmakeup gases, passes again through unfired material to dry and preheatthe same.

4. The. method defined in claim 3 wherein the makeup gases are suppliedat least in part through fuel burners.

5. The method defined in claim 4 wherein the volume of combined make-upand recycled gases used to dry and preheat the unfired material exceedsthe volume discharged to the atmosphere.

6. In the method for the continuous high temperature firing of apredominantly inorganic material wherein the material is raised to amaximum temperature in excess of 1,600 F. in a firing zone by theburning of fuel and wherein hot gases from the firing zone are thenpassed through unfired material in drying and preheating zones topreheat said material and at least partially cool said gases, the stepsof:

a. recycling the gases so used in the drying and preheating zones backthrough the firing zone and exhausting at least a portion thereof athigh temperature from the firing zone to the atmosphere wherebycombustible pollutants in the recycled gas are incinerated in the firingzone before discharge thereof to the atmosphere.

7. The method defined in claim 6 wherein the firing zone is maintainedbetween the top and bottom of a shaft furnace and the preheating anddrying zones are above the firing zone and there is a body of firedpellets in a cooling zone below the firing zone, characterized by thesteps of circulating gases from the top of the shaft furnace into thecooling zone near the bottom of the furnace and the gases which areexhausted to the atmosphere are withdrawn from the firing zone below thelevel of the preheating and drying zones, while combustion gases fromthe burning of fuel with air heat the material in the firing zone andsuch combustion gases combine in part with the recycled gases beingexhausted and in part with the gases which are so circulated from thetop of the furnace to the cooling zone near the bottom.

8. The method defined in claim 6 wherein the firing zone is maintainedin a bed of material on a traveling grate on one end of which the greenmaterial to be fired is charged to form a continuous layer and from theother end of which the fired product is discharged with drying andpreheating zones being provided along the grate between the charging endand the firing zone and with a cooling zone being provided along thegrate between the firing zone and the discharge end of the grate andwherein:

a. combustion gases generated in the firing zone by the burning of fuelare at least in part forced through the bed of material on the grate inthe drying and preheating zones and then recycled through the bed of hotmaterial in the firing zone to effect the incineration of combustiblecontaminants in the recycled gases and to mix with the newly generatedcombustion gases, and

venting a portion of the mixed newly generated and recycled gases to theatmosphere from the firing zone. 1

9. The method defined in claim 8 in which th material being processed issintered and wherein the raw material combined with fuel is continuouslyformed into a bed at the charging end of a traveling grate and carriedby the grate toward a discharge end first through an ignition zone toignite the fuel in the uppermost portion of the bed and then carriedover a succession of windboxes by which air is drawn through the bed tosustain combustion and a progressively deepening flame front ismaintained in the bed from the charging end toward the discharge end toprogressively sinter the material downwardly from the top of the bed andwherein the hot products of combustion passing through the bed below theflame front dries and preheats the raw material below the flame front inwhich:

a. all of the products of combustion and air drawn into said series ofwindboxes and utilized to dry and preheat the raw material in the bedare circulated downdraft through the bed adjacent the discharge end ofthe grate where most of the bed has been fired and is close to themaximum temperature of firing and all of the bed still remaining underthe flame front has been dried and preheated, and at least a portion ofthe gases and air so circulated through the bed near the discharge endare then exhausted to the atmosphere.

10. A thermal processing apparatus of the type used for firing asubstantially inorganic material to indurate, sinter or calcine the rawmaterial wherein the material is moved progressively in continuousprogression through drying, preheating and firing zones and offgasesfrom the firing zone are circulated through the raw material in thedrying and preheating zones characterized by the provision of means forconducting gases so used in the drying and preheating zones back to thefiring zone to effect incineration of combustible pollutants which saidgases acquire in the drying and preheating of the raw material, andfurther characterized by the provision of means for exhausting at leastsome of the hot gases so recycled directly from the firing zone to theatmosphere.

11. Apparatus as defined in claim 10 in which there is also a coolingzone through which the material is moved after it leaves the firing zonewherein means is provided to circulate at least some of the off-gasesfrom the firing zone which are used for drying and preheating the rawmaterial through the fired material in the cooling zone in beingrecycled to the firing zone.

12. The invention as defined in claim 11 in which the apparatus is ashaft type of furnace having an enclosed top with an air-lock meansthrough which raw material is charged into the top of the furnace andsaid means for conducting off-gases from the firing zone after they havebeen used to dry and preheat raw material comprises a duct leading fromthe enclosed top of the shaft furnace to the lower portion of the shaftfurnace, and fuel burning means is provided above said lower portion andbelow the top of the furnace and the cooling zone is between the firingzone and the lower end of the furnace.

13. The invention defined in claim 11 in which said apparatus comprisesan endless grate having a charging end and a discharge end and havinghoods thereover and windboxes thereunder establishing drying,preheating, firing and cooling zones with a duct for transferring hotgases from the windboxes of the firing zone into and through the rawmaterial on the grate in the drying and preheating zones, said means forrecycling combustion gases from the drying zone comprising a duct intowhich said gases, after passing through the bed of

2. The process defined in claim 1 wherein the portion of the gases sodischarged to the atmosphere is removed directly from an area of the bedwhere the temperature is above 1,600* F.
 3. The method defined in claim2 in which a portion only of the recycled gas is discharged to theatmosphere and the remainder, mingled with hot make-up gases, passesagain through unfired material to dry and preheat the same.
 4. Themethod defined in claim 3 wherein the make-up gases are supplied atleast in part through fuel burners.
 5. The method defined in claim 4wherein the volume of combined make-up and recycled gases used to dryand preheat the unfired material exceeds the volume discharged to theatmosphere.
 6. In the method for the continuous high temperature firingof a predominantly inorganic material wherein the material is raised toa maximum temperature in excess of 1,600* F. in a firing zone by theburning of fuel and wherein hot gases from the firing zone arE thenpassed through unfired material in drying and preheating zones topreheat said material and at least partially cool said gases, the stepsof: a. recycling the gases so used in the drying and preheating zonesback through the firing zone and exhausting at least a portion thereofat high temperature from the firing zone to the atmosphere wherebycombustible pollutants in the recycled gas are incinerated in the firingzone before discharge thereof to the atmosphere.
 7. The method definedin claim 6 wherein the firing zone is maintained between the top andbottom of a shaft furnace and the preheating and drying zones are abovethe firing zone and there is a body of fired pellets in a cooling zonebelow the firing zone, characterized by the steps of circulating gasesfrom the top of the shaft furnace into the cooling zone near the bottomof the furnace and the gases which are exhausted to the atmosphere arewithdrawn from the firing zone below the level of the preheating anddrying zones, while combustion gases from the burning of fuel with airheat the material in the firing zone and such combustion gases combinein part with the recycled gases being exhausted and in part with thegases which are so circulated from the top of the furnace to the coolingzone near the bottom.
 8. The method defined in claim 6 wherein thefiring zone is maintained in a bed of material on a traveling grate onone end of which the green material to be fired is charged to form acontinuous layer and from the other end of which the fired product isdischarged with drying and preheating zones being provided along thegrate between the charging end and the firing zone and with a coolingzone being provided along the grate between the firing zone and thedischarge end of the grate and wherein: a. combustion gases generated inthe firing zone by the burning of fuel are at least in part forcedthrough the bed of material on the grate in the drying and preheatingzones and then recycled through the bed of hot material in the firingzone to effect the incineration of combustible contaminants in therecycled gases and to mix with the newly generated combustion gases, andb. venting a portion of the mixed newly generated and recycled gases tothe atmosphere from the firing zone.
 9. The method defined in claim 8 inwhich the material being processed is sintered and wherein the rawmaterial combined with fuel is continuously formed into a bed at thecharging end of a traveling grate and carried by the grate toward adischarge end first through an ignition zone to ignite the fuel in theuppermost portion of the bed and then carried over a succession ofwindboxes by which air is drawn through the bed to sustain combustionand a progressively deepening flame front is maintained in the bed fromthe charging end toward the discharge end to progressively sinter thematerial downwardly from the top of the bed and wherein the hot productsof combustion passing through the bed below the flame front dries andpreheats the raw material below the flame front in which: a. all of theproducts of combustion and air drawn into said series of windboxes andutilized to dry and preheat the raw material in the bed are circulateddowndraft through the bed adjacent the discharge end of the grate wheremost of the bed has been fired and is close to the maximum temperatureof firing and all of the bed still remaining under the flame front hasbeen dried and preheated, and b. at least a portion of the gases and airso circulated through the bed near the discharge end are then exhaustedto the atmosphere.
 10. A thermal processing apparatus of the type usedfor firing a substantially inorganic material to indurate, sinter orcalcine the raw material wherein the material is moved progressively incontinuous progression through drying, preheating and firing zones andoff-gases from the firing zone are circulated through the raw materialin the drying and preheating zones characterized by the provision ofmEans for conducting gases so used in the drying and preheating zonesback to the firing zone to effect incineration of combustible pollutantswhich said gases acquire in the drying and preheating of the rawmaterial, and further characterized by the provision of means forexhausting at least some of the hot gases so recycled directly from thefiring zone to the atmosphere.
 11. Apparatus as defined in claim 10 inwhich there is also a cooling zone through which the material is movedafter it leaves the firing zone wherein means is provided to circulateat least some of the off-gases from the firing zone which are used fordrying and preheating the raw material through the fired material in thecooling zone in being recycled to the firing zone.
 12. The invention asdefined in claim 11 in which the apparatus is a shaft type of furnacehaving an enclosed top with an air-lock means through which raw materialis charged into the top of the furnace and said means for conductingoff-gases from the firing zone after they have been used to dry andpreheat raw material comprises a duct leading from the enclosed top ofthe shaft furnace to the lower portion of the shaft furnace, and fuelburning means is provided above said lower portion and below the top ofthe furnace and the cooling zone is between the firing zone and thelower end of the furnace.
 13. The invention defined in claim 11 in whichsaid apparatus comprises an endless grate having a charging end and adischarge end and having hoods thereover and windboxes thereunderestablishing drying, preheating, firing and cooling zones with a ductfor transferring hot gases from the windboxes of the firing zone intoand through the raw material on the grate in the drying and preheatingzones, said means for recycling combustion gases from the drying zonecomprising a duct into which said gases, after passing through the bedof material on the grate in the drying zone, are first conducted throughthe bed of fired material in the cooling zone, said cooling zone havinga duct for then circulating said gases to the firing zone, and means forexhausting some gases from the firing zone to the atmosphere.